1. Field of the Invention
This invention relates to a torque detecting device. More particularly, it is a device which can, for example, be used for converting the steering torque of an electric power steering system to an axial force, detecting its magnitude and direction and transmitting an output to an electric motor to control its driving force.
2. Description of the Prior Art
Devices for detecting the steering torque of an electric power steering system are disclosed in Japanese Laid-Open Utility Model Specifications Nos. 171670/1980 and 171671/1980. One type of device includes an input shaft connected to a steering wheel and having a lower end portion received in the upper end portion of a sleeve having a lower end portion splined about the upper end portion of an output shaft connected to a steering gear. The sleeve is held in position by a spring. The upper end portion of the sleeve has an axially inclined guide groove and the lower end portion of the input shaft has a guide pin received in the guide groove.
According to another type of device, an input shaft which is connected to a steering wheel has a lower end portion fitted rotatably in the upper end portion of an output shaft and having an axially straight slit, while the upper end portion of the output shaft has an axially inclined slit. A guide pin extends through the two slits and is connected to a ring fitted loosely about the output shaft. The ring is held in position by a spring.
If a steering torque is applied to the input shaft, the guide pin moves along the guide groove or the two slits and cooperates with the guide groove or the slits to produce a cam action which moves the sleeve or the ring axially of the shaft. Two switches are provided about the sleeve or the ring for detecting the axial movement thereof and closing a driving circuit for an electric motor to control its rotation.
As the cam action is produced by the sliding contact of the guide pin with the surface of the sleeve defining the guide groove or the surface of the output shaft defining the slit, however, the guide pin is likely to stick to the sleeve or the output shaft and thereby prevent the smooth movement of the sleeve or the ring, resulting in the vibration of the motor.
An improved device is, therefore, proposed in Japanese Laid-Open Patent Specification No. 177773/1983. This device includes a stepped shaft connected to an input shaft and having a diametrically enlarged portion provided with an inclined groove or a roller. A sleeve is connected to an output shaft rotatably therewith and vertically slidably. The sleeve has an inner surface provided with a roller which is slidably received in the groove on the diametrically enlarged portion of the stepped shaft, or an inclined groove in which the roller on the stepped shaft is slidably received. If a steering torque is applied to the input shaft, the roller on the stepped shaft or on the sleeve moves about the axis of the shaft along the inclined groove and thereby produces a cam action which moves the sleeve axially of the shaft. A potentiometer detects its axial movement and outputs a torque signal to control an electric motor.
The roller in the improved device receives so small a sliding resistance that it hardly sticks to the sleeve or the stepped shaft. Therefore, it is virtually unlikely that such sticking may prevent the smooth movement of the sleeve. The roller is, however, engaged in the groove for limiting the vertical movement of the sleeve, and for transmitting the rotation of the input shaft to the output shaft when the motor has failed. In other words, the roller itself defines a manual locking member. Therefore, a large external force is likely to act on the roller and deform, crack, or otherwise damage it. As a result there is every likelihood of the sleeve failing to move smoothly.